The starting positions are not purposely nerfed, BUT the Map Generator produces more less valuable starting positions on Deity than on Settler, how is that?
The Map Generator will generate all starting positions for all players before placing the individual player. When it places a player it does so pretty much randomly over the still available starting positions (this is not perfectly true for some scripts but we'll disregard this here ). Now when it places the human player it looks at the difficulty and on settler it will only accept the first starting position it rolls if it is within the top 10% of starting positions available, if not it goes on to the next and so on. On Deity it will accept any starting position that is in the top 90% of starting positions - now this does not mean that very good starting positions are impossible, since it will accept the best starting position if it randomly assigns that to the player, but it will not seek for the best as on settler.